The alternation between different weft yarns in such a loom requires the operative alignment of the respective insertion nozzles in a certain sequence with the shed-forming means of the loom, e.g. as known from U.S. Pat. No. 4,326,565. The nozzles are generally mounted for this purpose on a mobile carrier such as a rocker arm which is pivoted on an extension of the batten or sley near one end of a guide channel formed by the warp-engaging heddles or by so-called confiners interleaved therewith (see, for example, our commonly owned application Ser. No. 331,922 filed Dec. 17, 1981, now U.S. Pat. No. 4,448,223 issued May 15, 1984). The mobile carrier is, of course, synchronized with the loom cycle so that the insertion of a pick, i.e. a section of the weft yarn, can be initiated as soon as the shed has been reformed. Generally, it is also necessary to provide each nozzle with an associated thread clamp which immobilizes a cut-off end of the corresponding weft yarn until the nozzle is activated by a blast of air under pressure to insert another pick.
Thread clamps such as those here considered include a fixed and a movable jaw, the latter being conventionally operable by a single-acting pneumatic jack to which air under pressure is applied when the clamp is to close around the weft yarn. When the air pressure is relieved, a countervailing biasing spring reopens the clamp. When the clamp-operating air and the weft-injecting air are derived from the same source, such as a compressor or an accumulator, the air pressure available for holding one of the clamps closed is subject to variation as another nozzle is activated during that period; this could lead to a premature release of an engaged weft yarn which might then become entangled with the one selected for insertion. Moreover, the reopening of the clamp by means of a biasing spring is somewhat sluggish so that the release of the engaged weft yarn occurs with a certain delay after the air pressure has been relieved.